Raster centering control



April 24, 1956 L. DI ETCH RASTER CENTERING CONTROL Filed March 4, 1954 United States Patent O RASTER CENTERING CONTROL Leopard Dietch, Pennsauken, N. J., assiguor to Radio Corporation of America, a corporation of Delaware Application March 4, 1954, Serial No. 414,211

The terminal years of the term of the patent to be granted has been disclaimed 9 claims. (ci. sis- 27) This invention relates to television deflection systems and more particularly to means for centering a raster scanned by an electron beam in a cathode ray tube.

In present television practice, the picture is formed in a cathode ray tube, or kinescope, by an electron beam which is deflected to trace successive lines forming a raster on a uorescent screen. Deflection of the electron bean is accomplished magnetically by applying a sawt'ooth wave of current to the windings of a deflection yoke mounted about the neck of the cathode ray tube. Saw ,tooth deflection waves, generated by appropriate circuits in the television receiver, are amplified by a power amplifier o'utpt tube and are impressed upon an outputl transformer and thence upon the windings of the deflection yoke.

The direct current for the anode of the output tube which is' coupled to a B+ voltage source Hows through a path including a portion of the output transformer 'Win ing and a damper tube circuit. This direct current flow has a detrimental effect Vof saturating the magnetic core material of the output transformer, and results in the necessity of increasing the size of the transformer core and thereby increasing the cost.

Centering of the scanned raster may be accomplished by controlled flow of direct current in the deflection yoke. Direct current flow through the output transformer may further saturate the transformer core or may tend to desaturate the core', depending upon the direction of fow with respect to the flow of anode current from the out'- put tube. It is desirable to permit direct current to fioW through the yoke in either direction as dictated by the centering requirements. On the other hand, direct current flow through the output transformer should be Iim ited to one direction only, to minimize adverse saturation effects.

It is an object of this invention to provide an improved means for insertion of centering current into a deflection yoke without adversely affecting saturation of the output transformer.

It is a further object of this invention to provide a circuit employing the anode current of the output and damper tubes to accomplish raster centering, and thereby .eliminate the need for a separate direct current source.

Another object is to provide a novel circuit which will permit direct current flow in either direction through the deiection' yoke, but which will limit the direct current flow to a single direction through the output transformer.

According to this invention, a portion of the output transformer, an inductive choke, a potential dividing rcsistor and a balancing resistor are interconnected into a closed loop, thereby forming a direct current bridge circuit. Direct current is applied across the potential dividing' resistor forming one side of the bridge, and across theI balancing resistor, the transformer winding and the choke forming the other side of the bridge. The deflection yoke windings are connected across the bridge from the point between the transformer and the choke to the variable contact of the potential dividing resistor. v Centering current is permitted to flow in either direction through the deflection winding, but through the output transformer it is limited to the single direction in which its saturation effect upon the transformer core tends to cancel, or buck out, the saturation resulting from the flow of anode current from the output tube through a portion of this transformer to the damper tube.

Direct current is obtained from the power supply through the damper tube andan inductive linearity control that is open-circuited at its center for connection to the D. C. bridge. By the use of capacitors the linearity control is coupled for A. C. transfer at its center toy the low voltage ends of the yoke and the output transformer.

Other objects and advantages of this invention will become apparent upon a reading of the following specification and an inspection of the accompanying drawings iii-Which:

Figure l shows a circuit diagram, partially in blocks, of a television receiver including a horizontal deflection circuit made in accordance with the teachings of this invention; and

Figure 2 illustrates an equivalent direct current bridge network showing the direction of current flow through the various circuit elements.

In the two figures of the drawing similar elements are designated by the same numeral.

Referring to Figure l in more detail, there is shown a circuit for a televisionl receiver including an antenna 11 coupled to a television si-gnal receiving circuit 13. The television signal receiving circuit 13 includes a radio fre quency amplifier, a converter section, an intermediate frequency amplifier and a detector. A sound channel is indicate'd by the speaker symbol 15. A video amplifier 17 receives a composite signal and supplies a video signal to the control grid of a kinescope 19. A synchronizing signal separating circuit 21 supplies vertical synchronizing pulses tothe vertical synchronizing circuit 23 and horizontal synchronizing pulses to the horizontal deflection Wave generating circuit 25. Sawtooth deflection waves from the horizontal deflection wave generating circuit 25 are impressed on the control grid of a horizontal output tube 27 by a capacitive coupling 29. A g'nd resistor 31 provides the control grid with a proper bias. The screen grid s coupled to a source of direct energizing potential (not shown.) by a resistor 33 which is bypassed to ground by a capacitor 35.

Deflection waves of current from the output tube 27 are impressed upon the output transformer 37 and thence upon vthe horizontal winding 38 of the deflection yoke 39. Oscillations which would appear in the output transformer 37 are damped out by a damper tube 41 which is coupled to the B+ voltage supply through a linearity control 43. High voltage pulses appearing in the output transformer are rectified by a high voltage rectifier tube 45 and are stored by alcapacitor 47 to provide the ultor of the kinescope 19 with a'continuous high voltage. The output transformer 37 is essentially an autotransformer, but the low voltage portion of its winding 49 is isolated `from the flow of direct current from the high voltage portion of the winding 51 by a capacitor 53. The anode current for the output tube Z7 flows through the'winding '51, thence through the damper tube 41 and thence through thel linearity control 43 to a source of direct energizing potential (not shown). Three capacitors 55, 57 and 59 permit the flow of alternating current between the low voltage end of the deflection yoke 39, the low voltage end of the output transformer 37 and the mid-point of the linearity control 43.

The linearity control 43 is divided into two parts 43a, 43b which are coupled for alternating current flow by capacitors 57 and E39.y The direct of the damper tube 41 must ow from the center of the linearity control 43 through two parallel connected paths which form a bridge circuit. A potential dividing resistor or potentiometer 61 is onejpath for direct Ycurrent-110W and is one side ofthe bridge. The second path, constitutingfthe other side of the bridge, comprisesv a bal- Y .ancing resistor 63, the low voltage winding 49 of the output transformer 37 and an inductive choke winding 64. The horizontal winding of the deflectionryoke V39 is coupled across the bridge connections with a pointv 65 between the transformer winding 49 and the choke 64, and the adjustable contact of the potentiometer 61.

The bridge network` may be better understood by referring to Figure 2 which is an equivalent direct current circuit of a portion of the deflection circuit illustrated in Figure 1. Since Figure 2 deals only with direct currents,

currentV to the anode 49 respectively, and I'r is the direct current through Re..

only the resistive components of the circuit elements-areV shown. y Y

It will be appreciated that the anode current to the damper tube 41 flows from the B-jsupply through two parallel paths. The rst path includes the potentiometer 61. The second path includes the resistance Rs representative of the resistive Yvalue'of the inductive choke 64, a resistance Rr representative of the resistive value of the transformer winding 49 and the centering balance resistor 63. A resistor RY represents theresistive value of the horizontal winding of the detlection yoker39 and is con-V may be in either direction and may be varied considerably in value by varying the movable contact of the potentiometer 61.

The center balancing resistor 63 is equal in value to the difference between the resistive values of the inductive choke 64 and the transformer winding 49. i Ity functions Vto equalize the arms of the direct current bridge as shown in Figure 2. Since this resistor 63 lis bypassed by the capacitor 55, it has little effect upon the ow of alternating current in theroutput transformer 37.

For full appreciation of this invention the .various circuit elements may be considered to have the following sample values:

Horizontal windings of the deflection yoke 39: l2 milliy henries inductance: Ry=7 ohms D. C. resistance. Inductive choke 64: 120 millihenries inductance; Rs=50 ohms D. C. resistance Winding 49 of output transformer 37: 250 turns; Rf=l3 ohms D. C. resistance Winding 51 (from output tube to damper tube): 200

turns Centering balance resistor: RAL-'37 ohms Potentiometer 61: Rc=250 ohmsV A From the above values, the maximum available centering current in the yoke IY, for the extreme positions of the movable contact of the potentiometer 61 maybe cal- Y culated in terms of the output and damper tubes-anode current, IP, as follows:

. tiometer. being connected to a second terminal of said Then .Y

' c IY (mn) g-T-i IP m The direct current cancellationV effect, INnc, upon th output transformer may'be calculated as follows:

INDc=N51'IYP-N49'IT=0 A For'complete cancellation, where N51 and N49 represent the number of turns of the transformer windings 51 and lNDc=200 lP-250 .56 IP= IP Thus it is seen that with the assumed sample values, the ampere turns of saturation effect upon the output trans- 'former may be reduced to 30% of its initial rvalue before the direct current-cancellation. Further cancellation can be obtained by increasing the resistive value of the potentiorneter 61.

Although the foregoing specification and accompanying drawing teach the use of a novel bridge circuit employing direct current flow from the output tube and damper tube, it will be appreciated that a separate direct current source may be used with this'bridge circuit to achieve the same result. Y

for direct current and is one arm of the direct current bridge circuit, it may alsoV be used as a raster width coni trol. If this inductance is made variable between appropriate values, the loadingrof the output transformer 37 may be varied and the amplitude of the sawtooth deection wave as applied to the Vwindings of the deection yoke 39 may be controlled, thereby controlling the width of the scanned raster. Y l

The invention claimed is: Y l l. In a television receiver, a deflection system comprising, means including a'transformer winding for generatirng. adeection wave, an` inductive element and a resistive element connected in series with said transformer winding to form a closed circuit, and a deflection winding connected between the junction of saidtransformer Winding and said Vinductiveelement and a point on said resistive element intermediate the terminals thereof.

2. In a television deflection system Vhaving a deilection yoke winding coupled to Van' output transformer, a raster centering control circuit comprising a potential dividing Vpotentiometer having two terminals and an adjustable contact, an inductive choke connectedbetweena first of said potentiometer terminals and a Ypoint of coupling betweensaid output transformer and a irst terminal of said deflection yoke winding, the second terminal of said potentiometer being coupled to a second terminal of said output transformerthe adjustable contact of said potendellection yoke winding, said potentiometerv being operative to control direct current llow in said deection yoke winding.

3. Ina television deflection circuit having a deflectionV yoke winding coupled to an output transformena bridge circuit for controllingdirect current llow in-said deection yoke winding, said bridge circuit comprisinganinductive choke winding, at least a portion of a winding of said output transformer and at least one resistor al1 mutually connected to form a closed loop, a source of directY currentcoupled mutually across said resistor and across said inductiverchoke winding and said. connected transformer winding, said deflection yoke winding being cou.

pled across said loop from the connection point between said transformer winding and said inductive choke Winding to an adjustable contact on said resistor.

4, In a television deflection system having an output While the inductive choke 64 functions` as a shunt path tube coupled to a direct current source by a direct current path, a circuit for providing centering current to a deection yoke winding, said circuit comprising a linearity control coupled into said direct current path, said linearity control having an inductive Winding which is open-circuited at its mid-point, a resistor coupled across the opencircuited mid-point of said linearity control, said resistor having a movable contact for providing an adjustable flow of centering current to said deflection yoke winding.

5. In a television deflection system having an output tube coupled to a direct current source oy a path capable of passing direct current, a circuit for providing centering current to a dellection yoke winding, said circuit comprising a linearity control coupled into said direct current path, said linearity control having an inductive Winding which is open-circuited at its mid-point, a potentiometer coupled across the open-circuited mid-point on said linearity control, said potentiometer having an adjustable contact coupled to said deflection yoke Winding, a plurality of capacitors coupled across said potentiometer and said linearity control for bypassing alternating current around said potentiometer and across the open-circuited mid-point of said linearity control.

6. In a television deflection system, a circuit for providing centering current to a deflection yoke winding, said circuit comprising a direct current path from an anode of an output tube including an output transformer, a damper tube and an inductive linearity control, said inductive linearity control being open-circuited at its midpoint, a potentiometer coupled across the open-circuited mid-point of said linearity control, a plurality of capacitors coupled to said potentiometer and to said linearity control for passing alternating current across the opencircuited mid-point of said linearity control and around said potentiometer, said potentiometer having a variable contact coupling to said deflection yoke winding, said potentiometer being operable to adjustably control flow of direct current through said deflection yoke winding.

7. In a television deection system, a bridge circuit for providing' an adjustable ow of centering current through a deflection yoke winding, said bridge circuit comprising an inductive choke winding, at least a portion of a winding of an output transformer and a potentiometer, said choke winding, transformer Winding, and potentiometer being mutually connected to form a closed loop, a source of direct current coupledmutually across said transformer winding and said choke winding and across said potentiometer, said deflection yoke winding being coupled across said loop from a point between said transformer winding and saidchoke winding to an adjustable contact of said potentiometer.

8. In a system for deecting an electron beam to trace a raster in a cathode ray tube, a circuit for centering the raster in said cathode ray tube, said circuit comprising a means for generating deflection waves, an output tube coupled to said deflection Wave generating means, said output tube having an anode circuit capable of passing direct current, a deflection yoke winding adapted to be mounted in close spaced relation to said cathode ray tube, an output transformer coupled between the anode circuit of said output tube and said deflection yoke winding, a damper tube coupled to said output transformer, au inductive linearity control coupled between said damper tube and a source of direct current, the direct current path of said output tube anode circuit including at least a portion of said output transformer, said damper tube and said linearity control, said linearity control being opencircuited at its mid-point, a potentiometer coupled across the open-circuited mid-point on said linearity control, said potentiometer forming a first side of a direct current bridge, an inductive choke winding coupled to a second portion of said output transformer, said inductive choke winding and said second portion of the output transformer being coupled across the open-circuited mid-point y of said linearity control and forming a second side of said direct current bridge, said deflection yoke winding being coupled across said direct current bridge from the point between said inductive choke winding and said second portion of the output transformer to a movable contact of said potentiometer.

9. In a television receiver, a deflection system comprising, means including a transformer winding for generating a deflection wave, a deflection winding having one terminal connected to one terminal of said transformer winding and the other terminal coupled for alternating current transfer to the other terminal of said transformer winding, a resistive element having one terminal coupled to said other terminal of said transformer winding for direct current transfer, an intermediate tapping connected to said other terminal of said deflection Winding for direct current transfer and another terminal, and an element having a resistance component connected between said one terminal of said deflection winding and said other terminal of said resistive element to complete a direct current bridge circuit.

References .Cited in the le of this patent UNITED STATES PATENTS 2,579,627 Tourshou Dec. 25, 1951 2,580,977 Tourshou Jan. 1, 1952 2,586,521 Czeropski et al. Feb. 19, 1952 2,606,306 Bridges Aug. 5, 1952 2,612,622 Thalner Sept. 30, 1952 2,637,832 Rogers May 5, 1953 

